Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging”, can be performed by several methods.
In conventional wireline logging, a probe (or “sonde”) containing formation sensors is lowered into the borehole after some or all of the well has been drilled. The formation sensors are used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the instruments in the sonde through the conductive wireline. Conversely, the instruments in the sonde communicate information to the surface using electrical signals transmitted through the wireline.
An alternative method of logging is the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. “Measurement-while-drilling” (MWD) is the term for measuring conditions downhole concerning the movement and location of the drilling assembly while the drilling continues. “Logging-while-drilling” (LWD) is the term for similar techniques, which concentrate more on the measurement of formation parameters. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
In LWD systems, sensors typically are located at the lower end of the drill string. More specifically, the downhole sensors are typically positioned in a cylindrical drill collar positioned near the drill bit. While drilling is in progress these sensors continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. There are a number of existing and contemplated telemetry systems which may be used to transmit information regarding downhole parameters up to the surface without requiring the use of a wireline.
Of particular interest to the present disclosure are sensors for measuring standoffs, and logging instruments for measuring density. The term “standoff” refers to the distance between the borehole wall and the logging tool. A number of standoff measurement techniques exist in the oilfield industry. One example is a spring-loaded arm that extends from the tool to press against the borehole wall. The extension of the aim indicates the standoff measurement. Another example involves a piezoelectric transducer. The piezoelectric transducer transmits pulses of ultrasonic energy and measures the delay until echoes arrive from the borehole wall. With knowledge of velocity and transit time of the acoustic pulses, a standoff distance can be calculated.
One technique for measuring formation density involves the use of gamma rays. Gamma rays are high-energy photons emitted from an atomic nucleus. Such radiation is typically associated with the decay of radioactive elements. One example of an existing logging instrument for measuring density includes a gamma ray source of cesium-137. The gamma rays from the source travel into the formation where they interact with electrons. The interactions include absorption and scattering. Some of the scattered gamma rays return to detectors in the logging instrument where they are counted and their energy is measured. From the gamma ray measurements, a determination of electron density and lithology type may be made. From these determinations, a standard weight-to-volume density may be calculated.
Closely related techniques for performing porosity measurements involve the use of neutron tools. Neutrons emitted from a neutron source interact with the formation and are scattered back to detectors in the tool. From the detector measurements, a determination of formation porosity may be calculated. Accordingly, both gamma ray tools and neutron tools depend on measurements of radiation intensity.
Radiation intensity measurements are adversely affected by the downhole conditions, and in particular, are adversely affected by drilling fluid in the borehole and the relative geometry of the borehole. To compensate for these adverse effects, a pair of detectors may be included in the logging tool and their measurements combined. See, e.g., G. L. Moake, “A New Approach to Determining Compensated Density and Pe Values With a Spectral-Density Tool”, SPWLA Logging Symposium, paper 91-Z, 1991, which is hereby incorporated by reference. However, existing compensation techniques generally fail at larger standoff distances, so existing radiation intensity measurement tools are designed to remain in close contact with the borehole wall. A technique that compensates radiation intensity measurements made during large standoffs is desirable as it would allow for more versatile operation of the radiation intensity measurement tool.